The proposed study will address the hypothesis that insulin increases pyruvate utilization in rat adipose tissue in part by increasing the rate at which pyruvate is transported across the inner mitochondrial membrane. Data are presented which indicate that mitochondria isolated from insulin-treated fat cells and incubated in vitro with 14-C-labeled pyruvate utilize pyruvate more rapidly. After 5 seconds they also contain larger amounts of intramitochondrial labeled pyruvate than mitochondria isolated from control cells. Experiments are proposed to extend these observations and to provide unambiguous evidence for or against an action of insulin on the transport step. An important feature of the experimental approach proposed is its avoidance of the use of metabolic inhibitors in the assays of pyruvate transport. An attempt will be made to synthesize a radioactive analog of pyruvate which shares the transport carrier but is not metabolized. If the results support the hypothesis the effects of other hormones on pyruvate transport in fat cells will be investigated and comparative studies with liver mitochondria will be undertaken. The study should provide further clarification of the mechanisms by which insulin controls cellular metabolism and thus deepen our understanding of the metabolic derangements which accompany diabetes mellitus.